The scheme of the plumbing and wiring in a typical boat is shown above. An Ultraviolet Light based Water Purifier can be added as shown. The time delay is to allow for the striking delay of the UV lamp.

In practice, the scheme is likely to be complicated by the fact that both the terminals of the motor and pressure switch are unlikely to be independently available. One terminal would likely be grounded, connected to the mass of metal and pipework that stands for the "earth" in a boat. Modern practice is, I think, to ground the positive lead of the battery as this leads to less corrosion of the "hot" conductors since they will be negative.

If both the leads of the water pump are available then the purifier is connected to these terminals, and the motor connects to the delayed output from the delay circuit.

Perhaps it might be better to replace the delay with a light sensor that looks for the light from the UV lamp. That way, as soon as the lamp lights the motor runs.